Whispernet is Amazon’s way to allow you to wirelessly find and download content to your Kindle. If you have a 3G device you’ll use the 3G network (always free), otherwise you’ll need, and use, a Wi-fi network.
When you choose a book on the Amazon book store, add it to your cart and choose to buy it, it will automatically be added to your Whispernet queue. If you are on a 3G device, or you are attached to a Wi-fi network, your purchase will be delivered to you immediately (a book usually takes about 60 seconds to get to you in it’s entirity). If you are not connected to the Internet (e.g. you are on a Wi-fi only device, but there isn’t a network available) Amazon will look after your purchase until the next time you connect, at which time it will download it.
Whispersync is the ability to synchronize bookmarks, notes and furthest page read across Kindle devices and those running a Kindle app (for example an Apple iPhone).
This can be especially useful if you are reading a book across several different devices…Amazon looks after where you’ve got to, so all you need to do is pick up the device you wish to use, open the app or the book and hey presto, you’re right where you left off!